Asset Impairments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				3 Months Ended
	Dec. 31, 2014 MW		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2014 MW		Sep. 30, 2014 MW
Asset Impairments
Equity Method Investment, Ownership Percentage	50.00%					50.00%
Impairment losses	$ 97		$ 459	$ 0
Power Generation Capacity, Megawatts	52,000.00	[1]				52,000.00	[1]
Indian River
Asset Impairments
Impairment losses					459
Coolwater [Member]
Asset Impairments
Impairment losses						22
Power Generation Capacity, Megawatts	636.00					636.00
Gladstone Power Station
Asset Impairments
Equity Method Investment, Ownership Percentage	37.50%					37.50%
Impairment losses					92
Power Generation Capacity, Megawatts	1,613.00					1,613.00
Noncontrolling Interest, Ownership Percentage by Parent			37.50%		37.50%
Osceola facility [Member]
Asset Impairments
Impairment losses								60
Power Generation Capacity, Megawatts								463.00
Solar Panels [Member]
Asset Impairments
Impairment losses								$ 10

[1] Includes 95 active fossil fuel and nuclear plants, 14 Utility Scale Solar facilities, 35 wind farms and multiple Distributed Solar facilities. All Utility Scale Solar and Distributed Solar facilities are described in megawatts on an alternating current basis. MW figures provided represent nominal summer net megawatt capacity of power generated as adjusted for the Company's owned or leased interest excluding capacity from inactive/mothballed units.